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When a digital image of a uniform radiation source is obtained with a scintillation camera, the action of a variety of cortical receptive fields employed by the human observer can be simulated with a computer. A frequency diagram of receptor output can be generated by scanning the simulated receptor (which includes both exitory and inhibitory weightings of local image intensity). Although the measured frequency distributions exhibit a generally Gaussian shape, the dispersions are larger than predicted by Poisson statistics. The quadratic difference between measured dispersions and those predicted with Poisson statistics yields a measure of the texture contribution to noise that is interpretable in terms of small signal detection theory.